In general, mass spectrometers are configured so that ions generated from a sample component are separated according to their mass-to-charge ratios in a quadrupole mass filter or similar mass analyzer placed in a high-vacuum atmosphere and the separated ions are detected by an ion detector. FIG. 3 is a schematic configuration diagram of a common type of quadrupole mass spectrometer.
An almost hermetically sealed vacuum chamber 1 contains an EI ion source 2, an ion lens 3, a quadrupole mass filter 4, an ion detector 5 and other elements arranged along an ion beam axis C. When a gas containing a sample component is introduced into the EI ion source 2, the sample component comes in contact with thermions and is ionized. The generated ions are extracted from the EI ion source 2 and focused by the ion lens 3, to be introduced into the quadrupole mass filter 4 consisting of four rod electrodes. A preset amount of voltage composed of a radio-frequency voltage superposed on a direct-current voltage is applied from a power source (not shown) to each of the rod electrodes of the quadrupole mass filter 4. Only an ion having a mass-to-charge ratio corresponding to the voltage passes through the quadrupole mass filter 4 and reaches the ion detector 5.
As the ion detector 5, a secondary electron multiplier (EPM) is commonly used (see Patent Literature 1). FIG. 4 is a schematic configuration diagram of an ion detector 5 using a secondary electron multiplier. Each of the dynodes 51-56 arranged in a cascade structure is supplied with a preset amount of voltage from a power source 58. When an ion which has come from the left side on the figure and passed through the ion entrance 50 collides with the first conversion dynode 51, secondary electrons are emitted, which are then accelerated by an electric field and made to collide with the second dynode 52, which in turn emits a larger amount of secondary electrons. Such a process is repeated at each of the dynodes 53 . . . arranged in the cascade form, with the result that the secondary electrons are eventually multiplied to a significantly large number. The secondary electrons thus multiplied are made to enter a collector 57, and an electric current thereby generated in the collector 57 is extracted through a signal cable 59 as the detection signal.
To ensure high performance with the previously described mass spectrometer, it is necessary to maintain the inside of the vacuum chamber 1 at the highest possible degree of vacuum. In many cases, this is achieved using a vacuum pump consisting of the combination of a high performance turbomolecular pump (TMP) and a rotary pump for decreasing the back pressure for the TMP (see Patent Literature 2). Normally, as shown in FIG. 3, the turbomolecular pump 6 is directly connected to the vacuum chamber 1 in order to efficiently evacuate this chamber, while the rotary pump 7 is connected to the turbomolecular pump 6 via a connecting tube 8. The primary reason for separating the rotary pump 7 from the vacuum chamber 1 is to avoid internal contamination of the vacuum chamber 1 with the oil used in the rotary pump. By such a double-stage evacuation, the inside of the vacuum chamber 1 is maintained at low gas pressure (high degree of vacuum) within a range from 10−3 to 10−4 Pa.
As is commonly known, turbomolecular pumps perform evacuation in molecular units through interaction between moving blades (which are formed on a rotor rotated at high speeds) and stationary blades. Since the rotor is rotated at high speeds of up to several ten thousand rpm, turbomolecular pumps normally cause significant vibrations. In a system having a configuration as shown in FIG. 3, this mechanical vibration of the turbomolecular pump 6 is directly transmitted to the vacuum chamber 1, causing the various parts fixed to the vacuum chamber 1 to vibrate. The ion detector 5 is also one of the parts fixed to the vacuum chamber 1, which means that the mechanical vibration of the vacuum chamber 1 is also transmitted to the ion detector 5. The transmitted vibration may cause a noise in the detection signal extracted from the ion detector 5.
The primary reason for this noise is probably the vibration of a signal line (signal cable) extracted from the ion detector 5: When the signal line vibrates, the distance between the outer surface of the signal line and the vacuum chamber 1 or other surrounding members slightly changes, which causes a fluctuation in the impedance of the signal line. Since the preamplifier for amplifying detection signals from the ion detector 5 has a high input impedance, the fluctuation in the impedance of the signal line can easily appear as a noise. The frequency of the noise superposed on the detection signal due to the mechanical vibration of the turbomolecular pump is within a range from several hundred Hz through several kHz, which in some cases overlaps the frequency of the signal obtained by mass spectrometry. Therefore, it is difficult to electrically remove this noise using a filter or similar devices.